Autofocus systems presently used for microscopes can roughly be divided into two different classes. The first class includes so-called "active" autofocusing systems. These project at least one auxiliary light dot or mark onto the surface of the object being examined and evaluate the character of the light dot (e.g., its shape, size, position) to create a focusing criterion. Such active autofocusing systems have been described in German Pat. Nos. 3328821 and 3446727 and also in U.S. Pat. No. 4,639,587. These active systems are predominantly used in reflected light microscopes and have the advantage of working very fast, i.e., that they are able to quickly follow movements of the object. In addition, they possess a relatively large capture range, i.e., they are capable of focusing through a relatively deep field.
However, active systems are not readily suited for microscopic examinations of transmitted light objects and, specifically, of covered transmitted light objects. This is because, in the case of covered objects, the object plane itself does not produce a reflection. Instead, the reflection of the auxiliary light dot occurring on the cover glass is normally so strong that the autofocusing device will focus not on the object plane but on the cover glass surface. Further, the same problem can occur also with uncovered objects, since the actual object plane being observed often is not identical with the object surface but may lie within the object. While this off-object focus could be compensated by providing an additional focusing-motion adjustment varying in accordance with the cover glass thickness, such would be possible only for objectives having a relatively small lateral magnification factor. Objectives with larger lateral magnification factors have much smaller focal depths, so the distance between the cover glass surface and the object plane is often greater than the capture range that would normally be required for an active autofocusing procedure.
It would also be possible, when trying to focus on covered objects with such known active systems, to provide specific correction lenses for the auxiliary lighting of the autofocusing device. Such special lenses would be designed to have one focus position for the visible light used for observation of the object and another focus position for the auxiliary lighting device (which normally operates in the infrared range), and the lens design would separate these two focus positions by the cover glass thickness. However, even ignoring the expense of the special lens system, this solution would have serious disadvantages. For one, cover glass thickness is normally subject to fluctuations; and for another, wavelength-dependent focal differences vary from lens to lens so that, especially in the case of lenses with a large lateral magnification factor and small focal depth, such a system would be unable to effect sufficiently accurate focusing.
Further, active autofocusing systems are also often less than satisfactory (a) when used with uncovered objects which possess a very uneven surface and/or (b) when the surface is not identical with the object plane that is of interest. In such cases, reflection of the auxiliary light is dispersed and thus greatly weakened, preventing proper autofocusing, or the measuring dot may impinge on lands or holes in the surface of the reflected light object and thus focus on a plane that is not desired. A widening of the projected measuring dot offers no solution to this problem because the focusing will then average out on an intermediate plane which similarly does not necessarily correspond to the actual object surface.
Because of these just-mentioned limitations of active autofocusing systems, the second general class of autofocusing systems, which are characterized as "passive", are predominantly used in transmitted light microscopes. These passive autofocusing systems evaluate a video signal of the object's image. The key criterion for focusing such passive systems is normally the image contrast, which can be determined in various ways. Such passive autofocus systems are known in the art, e.g., see British Pat. Nos. 1,401,179 and 1,545,437 and U.S. Pat. No. 4,600,832.
However, prior autofocus systems of the passive type work rather slowly; and since image contrast does not contain any directional information, an initial "search pass" must be made before the automatic circuitry can find the point of maximum contrast. There are known passive systems, e.g., as shown in U.S. Pat. No. 3,883,689, which do obtain directional information by arranging two image sensors, respectively, before and after the image plane. However, since such systems require multiple, complex beam splitting, they double the intervention in the microscope's transmitted light path and, besides, need at least one additional camera just for the autofocusing, in addition to the camera used to develop the video signal of the image. Also, the expense of this solution is thus very high.
Another disadvantage of passive video autofocus systems lies in their relatively short capture range, i.e., the distance through which the automatic device focuses and/or performs the search pass. If the capture range is made too large, there is a danger that the automatic device will focus on so-called "parasitic planes", e.g., image planes of surfaces of optical components in the microscope, such as a dirty condenser lens, etc.
The problem underlying the present invention is the need to provide an autofocus system for microscopes which can perform effectively both for reflected light operation and also for viewing of covered and uncovered transmitted light objects and which can perform at high speed and with appropriate accuracy.